In the past, such a structure that, even in case that one of two computers, which are connected to one sub-network and communicate with each other, was moved to a different sub-network, these computers can communicate with each other, has been devised. As one example thereof, Internet protocol Mobile IP has been known, which is as described in IETF draft “draft-ietf-mobileip-ipv6-22.txt” and literature “Ipv6 Textbook” (under the editorship of Hiroshi EZAKI, IDG Japan publishing, Sep. 20, 2002, p84-86).
In addition, tried is realization of a mobile computer system which can communicate with a computer in a sub-network at home, or a computer in another network, by use of simple communication procedures, without changing parameters etc. of network environment at all, by connecting a computer which is connected to a sub-network at home and uses normal IP, to a remote network. As this example, there is a thing as described in JP-A-10-23076 publication. This is such a thing that, in order to enable a computer which was moved to another sub-network to be used in parameters of the same network environment as one which was connected to a sub-network at home, a special router, which is called as a VPN (Virtual Private Network) router, is disposed in advance, in another sub-network. It is designed that, by this means, (1) certification of a mobile computer and establishment of connection to a home sub-network, (2) accommodation of a mobile computer by use of address resolution protocol, are carried out, and then, (3) communication with a computer in a home sub-network, or (4) communication with a computer other than that in the home sub-network, is carried out.
In addition, there is also a system which was designed in such a manner that VPN setup service according to IP Sec tunnel between arbitrary terminals is provided, coordinating with location registration procedure in mobile IP, without giving a special function for VPN. In this system, it is composed of a mobile terminal device, a certificate server, a VPN database, and a network device, and a home certificate server extract VPN information of a user who requested certification at the time of requesting for location registration from the mobile terminal device, from the VPN database, and notifies the VPN information to each network device by use of predetermined location registration message and certification response message. The network device is designed to setup VPN paths according to IPSec, between a home network device and an external network device, between the home network device and a predetermined network device, and/or between the external network device and the predetermined network device, respectively, on the basis of the notified VPN information.
However, in the method as shown in the IETF draft “draft-ietf-mobileip-ipv6-22.txt” and the literature “Ipv6 Textbook” p84-86, there is a necessity to implement unique protocol to all terminal devices to be used, and therefore, there is such a problem that it can not be used in the conventional mobile terminal device.
In addition, in the communication system as shown in JP-A-10-23076 publication and JP-A-2002-44141 publication, it has such a problem that there is a necessity to have a terminal device executed a specific procedure at a destination.
The invention is made so as to solve the above-described conventional problem, and aims to provide a mobile communication system which can make the same communication as that before movement, even after it was moved, without disposing a special structure in a terminal device.